That era BMW's were very well put together. Many of the fitting like stainless clamps and other details are superior to newer models. The R1100GS engine is delightful power-plant, with great low end grunt and superior engine braking to the R1150. It also has a cable clutch which is good maintenance wise and will make your fingers stronger. Much heavier to pull than the hydraulic clutch, but you get used to it fast.
Areas of concern on that model is periodic spline lube, the original M94 transmission had some problems on certain bikes, upgrading to M97 specs is a common fix. Though many bikes like mine with 90k miles still have the original M94 and have no problems. Rewiring or replacing the hes sensor is likely necessary if it hasn't already been done.
In Australia there was a rash of broke transmission mounts and TT made the "hard part" fix If you plan to ride it hard heavily loaded in the outback that's a good idea to do the fix. American bikes which spent most of their lives on smooth road did not break often.
Wear items like brakes and shocks run into a fair amount of money, if you're on a tight budget keep those in mind. The stock seat pretty much sucks so bear that in mind also.
Once all the little particularities of that model have been sorted it makes for a very reliable, economical and pleasant bike to ride.
The best deals would be on a well cared for higher mileage bike where the previous owner has already sorted out all the little stuff.